1. Field of the Invention
The present invention relates to a hydraulic circuit for a working machine, such as a crushing machine, that has an attachment to which working devices are selectively attached.
2. Description of the Related Art
A typical crushing machine includes a base machine and an attachment which is mounted on the base machine and to which working devices, such as a compression crusher and a breaker, are selectively attached at an end of the attachment depending on the kind of work to be performed. In the following description, work using the breaker is called breaking.
In this case, the compression crusher and the breaker use different hydraulic actuators, and the actuators must be provided with respective oil supply/discharge passages. Accordingly, a hydraulic circuit must be switched depending on the kind of the working device to be used.
FIGS. 6A and 6B are diagrams illustrating a structure in which the circuit is switched manually. FIG. 6A shows a circuit state for compression crushing and FIG. 6B shows a circuit state for breaking.
Referring to FIGS. 6A and 6B, the structure includes a hydraulic pilot-operated directional control valve 5 operated by a remote control valve 6, a manual directional control valve (three-way valve) 7, a hydraulic pump 8 that functions as a hydraulic power source for actuators, a tank T, and a primary hydraulic power source 11 for the remote control valve 6. When the control valve 5 is operated, oil is supplied from the hydraulic pump 8 to a compression-crusher cylinder 9 or a breaker cylinder 10, and the compression-crusher cylinder 9 or the breaker cylinder 10 is operated accordingly.
As shown in FIG. 6A, when compression crushing is performed, similar to a double acting cylinder circuit, input and output ports of the compression-crusher cylinder 9 are connected to the hydraulic pump 8 and the tank T via the control valve 5.
When breaking is performed, the power of the breaker is reduced if a back pressure is applied to a return line of the breaker cylinder 10 due to a throttle effect of the control valve 5, and there is a risk that the breaker cannot be operated. In addition, pulsation occurs in an oil cooler (not shown) and there is risk that the oil cooler will be damaged.
Therefore, as shown in FIG. 6B, when breaking is performed, the directional control valve 7 is operated so that the return line of the breaker cylinder 10 is directly connected to the tank T.
A technique for automatically switching the circuit state with a directional valve depending on the kind of the working device without using the manual directional control valve 7 shown in FIGS. 6A and 6B is disclosed in Japanese Unexamined Patent Application Publication No. 2002-294758.
Also in this case, there is a risk that a fail, such as a breakage of a line connecting a controller and a solenoid valve or a breakdown of the controller, will occur in an electrical or hydraulic control system of the directional control valve that functions as an automatic control device. Accordingly, there is a possibility that the circuit state expected by the operator and the actual circuit state do not match.
Therefore, the following problems occur:
(i) If the actual circuit state is set to the state for compression crushing even though breaking (circuit state for breaking) is selected by a mode switch operation performed by the operator and breaking is performed, the back pressure in the return line is increased as described above and there is a risk that the power of the breaker will be reduced or the breaker will stop.
(ii) If the actual circuit state is set to the state for breaking even though compression crushing (circuit state for compression crushing) is selected by the operator and compression crushing is performed, the compression-crusher cylinder cannot be reciprocated and operates in only one direction since one of the lines of the cylinder is directly connected to the tank.